There have been a few times in the history of mankind when we nearly died out as a species. Anthropologists call these events “bottlenecks”, times when the population of humans shrank – perhaps to as few as 2,000 people over 50,000 years ago. At those levels, we would be categorised as an endangered species on the IUCN Red List, existing in even fewer numbers than wild tigers do today. We survived, and in fact we’ve thrived, mainly because we adapted our environment to suit our needs. But to what degree has the survival and triumph of our species changed our planet? The best people to answer this could well be those who have the grandest perspective. Geologists can take a 4.5-billion-year step back and look at the human impact on our planet in the context of its long history – they can identify changes in the rock record within layers of deposited sediments that build up and are compressed over time.